The ruler is only fitted for Jolla 1 and Xperia screen sizes. For the tablet it's already harder because of a different screen ratio. I will have another look at it.
But no plans to work on the favourites part at the moment.

As mentioned in the description: "Translations are only partial because values are used as reference in the code.".
Maybe it could be made fully translatable, but that would require more that a little rework.
For now it is partially translated to Dutch and French. You could download https://github.com/a-dekker/Unit-Converter/blob/master/translations/harb... and open it in Qt Linguist 5. Then you can choose your destination language and translate the translatable strings.

Unlike the Jolla store version this fork actually runs in SFOS 2.1.4.14! However it needs a lot of work at currency conversion. The list of currencies is long and unmanageable while being incomplete.
Here is a list of suggestions:
1 order the list of currencies alphabetically
2 add missing currencies, e.g Byelorussian rouble BYN.
3 fix favourites by allowing to enter a value to convert between bookmarked units AND allow to reverse the pairs.

Ad 1)
I will see how easy it is to sort the currencies alphabetically. I agree it would make searching easier.
Ad 2)
The currency info is from http://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml, so we are kind of limited to what they provide. Byelorussian rouble BYN is not part of that at the moment.
Ad 3)
"Fix" sounds like broken. This looks like a "nice to have". You can always swap the values yourself of course.

It turned out to be not that hard, as Qt.locale().decimalPoint can tell what sign is used. B.t.w. it does not work if you start the app from the command prompt.
So 2.8-1 should now support both comma and dot.

The orignal version translated all dots to commas and did not allow a dot as input. Not really convenient if your numeric keyboard has a decimal dot.
So I removed the dot to comma conversion (hoping it would display the appropriate separator) and allowed comma and dot as input. But that is not working out I presume.

I do think it's hard to solve, maybe some language like settings can tell what decimal sign is active for the numeric keyboard, then followed by some correcting actions.